The invention relates to a process to anneal non-ferrous metal objects in an adhesion-free manner, that is to say, to avoid so-called adhesions during the annealing procedure, especially in a bell-type furnace.
Non-ferrous metals such as, for instance, bronze wires or bronze strips undergo a homogenizing annealing step after the casting and shaping. Subsequently, additional shaping measures such as rolling or drawing are taken and re-crystallization annealing steps are carried out.
The annealing temperatures lie between 300.degree. C. and 700.degree. C. [572.degree. F. and 1292.degree. F.]. The annealing is conducted in continuous furnaces, a relatively complex approach in view of the fact that the objects usually have a small cross section.
When annealing coils, which can be done, for example, in bell-type furnaces, local diffusion welds, so-called adhesions, occur at the contact sites of the objects, for example, between individual windings of the wound-up wire or strip, due to diffusion mechanisms. Upon further processing, that is, during winding, these adhesions cause cracks on the surface of the material, thereby giving rise to surface defects. As a consequence, of course, adhesions are highly undesirable on annealed non-ferrous metal objects.
In this context, the term non-ferrous metals refers to alloys whose main components are copper, tin, aluminum and lead, whereby many other components are also possible such as, for instance, magnesium, nickel, etc.
In order to avoid adhesions when annealing steel strips, DE 4207394 discloses the approach of changing the water-gas equilibrium in a targeted manner in the presence of H.sub.2, CO.sub.2, CO and H.sub.2 O in the protective-gas atmosphere in such a way that a totally oxidizing atmosphere is available at the end of the holding phase, while a totally reducing atmosphere is available in the cooling phase. This approach, however, cannot be used for non-ferrous metals due to the much lower temperatures that are found at times, for instance, 400.degree. C. [752.degree. F.], and due to the detrimental effect on the part of the reaction products, such as CO and H.sub.2 O, on the oxidation mechanism.